Electric- or hybrid-powered machines are widely used in various industrial applications. A high voltage power system is required for driving these machines. Typically, the machine frame will be electrically isolated from the terminals or conductors of the high voltage electrical components in the high voltage power system.
Under normal working conditions, leakage currents exist between the conductors of the high voltage power supply and the machine frame. Thus, the insulation resistances between the conductors of the high voltage power supply and the frame are normally very high. However, under faulty conditions (e.g., insulation failure), electric currents from the high voltage power supply may leak to the machine frame. Such leakage currents, when significant, may be an indication of machine component fatigue or failure of a conductor's insulation. In order to ensure the proper operating conditions and the integrity of the machines, it is necessary to protect the components of the machines under these faulty conditions.
An exemplary system that may be used to protect components of an electric machine under ground fault conditions is disclosed in Canadian Patent 1266117 to Kumar et al. that was issued on Feb. 20, 1990 (“the '117 patent”). The system in the '117 patent includes ground fault responsive means activated when the magnitude of ground leakage current is abnormally high. Specifically, when the leakage current rises to a magnitude higher than a predetermined duration threshold level but not higher than a predetermined maximum permissible limit, the system reduces output of a power source providing electric power to the electric machine to a fraction of its normally desired amount. The '117 patent explains that the fraction is inversely proportional to the leakage current magnitude in excess of the threshold level. When the leakage current magnitude rises above the maximum limit, the system restricts the power output to zero for at least a predetermined time interval. At the end of that interval, the system automatically removes the zero-power restriction if the leakage current magnitude is then below a certain reset point (which is appreciably lower than the maximum limit).
Although the system of the '117 patent may be useful in protecting components under ground fault conditions, the system of the '117 patent unnecessarily restricts the power output to zero when the ground leakage current magnitude rises above the maximum limit. In certain applications, it is desirable to move the electric machine under ground fault condition to a work shop so that the power supply system can be fixed. However, when the power output is restricted to zero, the electric machine cannot be moved.
The ground fault protection system of the present disclosure is directed toward solving the problem set forth above and/or other problems of the prior art.